Hot subdwarfs are hots stars with masses somewhat less than that of the Sun. ... They are known as sdB stars because their spectra are dominated by hydrogen lines. ... Normally, a red giant will switch on core helium burning without losingthe layers of hydrogen around the helium core. sdOB stars are related to sdB stars, possibly being the sdB stars which have completed core helium burning and are evolving to become white dwarfs. ... Such stars are known as PG1159 stars and DO white dwarfs. ...